Atomizing-based cutting fluid delivery system and method

ABSTRACT

An atomizing cutting fluid system. The system includes a common chamber terminating in a shaped droplet nozzle and including a nozzle section immediately behind the shaped droplet nozzle. An atomizer creates spray directly within the common chamber behind the nozzle section. A cutting fluid supply line provides cutting fluid to the atomizer. A high velocity gas nozzle within the nozzle section and behind the droplet nozzle is configured to provide a high velocity gas to entrain the flow of droplets. The nozzle section and droplet nozzle are configured to produce a fully developed droplets-gas flow at a predetermined distance from the droplet nozzle. In a cutting system, the spray system provides a uniform film for a macro or micro cutting operation at sufficient flow rates.

PRIORITY CLAIM AND REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119 from priorprovisional application Ser. No. 61/830,262, which was filed Jun. 3,2013.

FIELD

A field of the invention is machining of metals and metal alloys. Theinvention provides an atomization-based system and method for creatingand applying a thin film of cutting fluid that can be used for coolingand lubrication in machining. An example application of the invention istitanium alloy machining.

BACKGROUND

Hard to machine materials, such as titanium and its alloys, producelocalized extreme temperatures during machining. This limits cuttingefficiency and also quickly wears out expensive tools. Short tool lifeleads to frequent interruptions in manufacturing, high maintenancecosts, and sometimes damage to an expensive workpiece being machined.Damage to a workpiece increases manufacturing defect rates and raisesexpense overall.

Titanium alloys are often used to produce complex and critical partsused, for example, in aircraft and medical implants. Additional exampleapplications include aerospace structures and engines, rockets,spacecraft, turbines, automotive engine components, nuclear and chemicalplants, petrochemical industries, offshore engineering, food processing,and biomedical devices. The alloys possess high strength-to-weightratio, high-temperature strength, strong fracture and corrosionresistance, and biocompatibility.

Titanium alloys are very difficult to machine, however, and tool life ispoor in systems that machine titanium. Titanium has poor thermalconductivity and low elongation-to-break ratio. Titanium is alsochemically reactive with typical tool materials at a cutting temperatureof 500° C. and above. As a result, highly-localized temperatures aredeveloped at the tool-chip interface. Severe edge chipping and plasticdeformation via galling and seizure of chips are often produced. Thisultimately shortens tool life, can be detrimental to surface finish, andcan cause parts to fail quality requirements.

Various efforts have been made to address these problems in machiningtitanium. One technique is known as flood cooling. See, e.g., Nandy, A.K., et al., “Some studies on high-pressure cooling in turning ofTi-6Al-4V,” International Journal of Machine Tools and Manufacture, 49:182-198 (2009); Cheng, C., et al., “Treatment of spent metalworkingfluids,” Water Research, 39: 4051-4063 (2005). The flood techniques areused in practice, despite relatively ineffectiveness and alsounfriendliness to the environment due to large quantities of toxicfluids used for cooling/lubrication.

High pressure cooling technique applies coolant at 70-160 bar or moredirectly at the tool/workpiece interface. A three to four-folds toollife increase compared to flood cooling has been reported by some. See,e.g., Nandy & Paul, “Effect of coolant pressure, nozzle diameter,impingement angle, and spot distance in high pressure cooling with neatoil in turning Ti-6Al-4V,” Machining Science and Technology, 12: 445-473(2008); Palanisamy, S., et al., “Effects of coolant pressure on chipformation while turning Ti6Al4V alloy,” International Journal of MachineTools and Manufacture, 49: 739-743 (2009). In practice however, overallproductivity improvements have been reported to be about 50%. The lowerproductivity improvement is attributable to a higher consumption rate ofthe cutting fluid, its delivery cost at such high pressure, and thesystem setup cost. Pusavec, F., et al., “Transition to sustainableproduction-Part I: application on machining technologies,” Journal ofCleaner Production, 18: 174-184 (2010).

Another difficult to implement process is cryogenic cooling. Whileoffering improved tool life, this is an energy-intensive process thatrequires liquid nitrogen (LN₂) to be delivered at high rates in therange of about 45-250 L/hr. Hong, S. Y., et al., “New cooling approachand tool life improvement in cryogenic machining of titanium alloyTi-6Al-4V,” International Journal of Machine Tools and Manufacture, 41:2245-2260 (2001). The liquid nitrogen delivery also poses safety risksto operators and other personnel.

With a goal of environmental friendliness, others have usedsupercritical CO₂ (scCO₂) as a solvent to dissolve cutting fluid.Clarens et al., “Evaluation of cooling potential and tool life inturning using metalworking fluids delivered in supercritical carbondioxide,” Proc. of the ASME International Manufacturing Science andEngineering Conference (MSEC), October 4-7, West Lafayette, Ind., USA(2009). In this method, CO₂ gas is provided at levels substantiallyabove its critical pressure, 72.8 bar. Tool wear rates realized duringmicro-machining were approximately equal to those of conventional floodemulsion systems. In experiments described in this paper, scCO₂ spraywas provided at 130 bar. These high pressures required a heavy andsophisticated system layout. The costs are prohibitive for such asystem, given the lack in improvement over the flood techniques. Also,high pressures pose safety risks to operators and the other personnel.Finally, only low cutting speeds of ˜45 m/min and depth of cut (0.27 mm)was reported, which would not be well-suited for macro-machiningapplications.

Efforts by some of the present inventors and colleagues have focused onatomized spray application of cutting fluids, and have proven to besuccessful in micro-machining applications. Micro-machining of AISI 1018steel with atomized cutting fluid droplets was demonstrated in Jun,Joshi, DeVor, and Kapoor, “An experimental evaluation of anatomization-based cutting fluid application system for micromachining,”ASME Transactions—Journal of Manufacturing Science and Engineering, 130:0311181-8 (2008). This system was limited to a flow rate of about 1mL/min, which is ill-suited toward macro-machining applications ingeneral, and also toward the more difficult materials, such as titaniumalloys. Macro-machining applications require machining at or above about1 mm depth of cut and 0.1 mm/rev or higher feed rate. This largercutting zone creates faster evaporation rates and, in the disclosedset-up, a small amount of cutting fluid can even evaporate prior toreaching the tool-workpiece interface.

Typical commercial nozzle units used in minimum quantity lubrication(MQL) systems employ a high-velocity gas to produce fluid droplets withshear mechanism. The size of fluid droplets varies in a wide range insuch systems. The fluid flow rate in these systems is typically limitedat ˜2-3 mL/min, a level that is insufficient for providing cooling andlubrication effect during machining at the macro-scale.

Machining of difficult materials, especially of materials havingproperties like titanium alloys, and especially at the macro-machininglevel, remains inefficient and expensive. Tools are replaced often andmachine surfaces can exhibit defects. Defects can compromise partintegrity and can cause a high part rejection rate, leading toadditional expense.

SUMMARY OF THE INVENTION

An embodiment of the invention provides an atomizing cutting fluidsystem. The system includes a common chamber terminating in a shapeddroplet nozzle and including a nozzle section immediately behind theshaped droplet nozzle. An atomizer creates spray directly within thecommon chamber behind the nozzle section. A cutting fluid supply lineprovides cutting fluid to the atomizer. A high velocity gas nozzlewithin the nozzle section and behind the droplet nozzle is configured toprovide a high velocity gas to entrain the flow of droplets. The nozzlesection and droplet nozzle are configured to produce a fully developeddroplets-gas co-flow at a predetermined distance from the nozzlesection. In a cutting system, the spray system provides a uniform filmfor a micro or macro cutting operation at sufficient flow rates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram of a preferred embodiment atomizingcutting fluid system of the invention;

FIG. 1B is a schematic diagram of a preferred embodiment atomizingcutting fluid system of the invention;

FIG. 1C is a schematic diagram of a preferred embodiment atomizingcutting fluid system of the invention;

FIG. 2 illustrates a preferred example embodiment mixing section for thehigh-velocity gas inlet of the systems of FIGS. 1A-1C along with exampledimensions to illustrate preferred example ratios to control respectivevolumetric pressures and flows of air and CO₂;

FIG. 3 is a partial perspective view of the nozzle section of the systemof FIG. 1A;

FIG. 4 is a partial cross-sectional view of a preferred nozzle sectionfor an atomizing cutting system consistent with FIG. 1A;

FIG. 5 illustrates an example cutting system including a cutting fluidsystem in accordance with the invention arranged to lubricate and cool acutting tool/workpiece interface;

FIGS. 6-7B illustrate an experimental set-up used to test a cuttingsystem and cutting fluid system of the invention;

FIGS. 8A-8D are two-way diagrams for the significant two-factorinteraction effects on tool life;

FIGS. 9A-9D are images of experimental cutting fluid sprays produced inexperiments with different combinations of droplet and gas velocities;

FIG. 10 is a droplet flow velocity diagram in the entrainment zone ofthe spray;

FIGS. 11A and 11B together and FIGS. 11C and 11D together respectivelyillustrate schematic diagrams for a larger and a smaller dropletimpingement angles; and

FIG. 12A schematically illustrates a fluid spray and FIGS. 12B-12Dschematically illustrate spray cross section at near, intermediate andfar-field locations.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present inventors have determined that a limiting factor of flowrates in previous ACF spray systems for micro-machining is the deliveryof atomized fluid droplets from outside of the chamber, typically by along pipe into the machine chamber. This limits the flow rate to about1-2 mL/min from the outside of the machine chamber to the inside. See,e.g., Jun, M. B. G. et al., “An experimental evaluation of anatomization-based cutting fluid application system for micromachining,”ASME Transactions, Journal of Manufacturing Science and Engineering,130: 0311181-8 (2008). Others have also studied ACF for micro machining.See, e.g., Rukosuyev, M. et al., “Understanding the effects of systemparameters of an ultrasonic cutting fluid application system formicromachining,” Journal of Manufacturing Processes 12/2: 92-98 (2010).One problem that the present inventors have recognized is that it isdifficult or impossible to deliver droplets from outside at higher fluidrates, e.g., 10-20 mL/min that are required for macro-scale machiningdue to droplet-droplet and droplet-wall interactions.

Reduced film thickness and faster droplet spreading with increasing jetpressure has been investigated in the context of lubrication of internalcombustion engines. See, e.g., Stanton & Rutland, “Multi-dimensionalmodeling of thin liquid films and spray-wall interactions resulting fromimpinging sprays,” International Journal of Heat Mass Transfer, 41:3037-3054 (1998). Others have studied lubrication and confirmed thatlubrication will be effective when there is a stationary surface and amoving surface with a film in between. See, Langlois, W. E., “AWedge-Flow Approach to Lubrication Theory,” Quarterly of AppliedMathematics 23:39-45 (1965).

After a certain level of jet pressure, higher incident velocity isinduced and will result in droplets splashing instead of effectivelyspreading upon impingement. See, Yarin & Weiss, D. A., “Impact of dropson solid surfaces: self-similar capillary waves, and splashing as a newtype of kinematic discontinuity,” Journal of Fluid Mechanics, 283:141-173 (1995). The impingement angle with respect to the base surface(i.e. tool rake) defines the regime for sticking the droplets,otherwise, partial rebound or split deposition will take place. Chen &Wang, “Effects of tangential speed on low-normal-speed liquid impact ona non-wettable solid surface,” Experiments in Fluids 39: 754-760 (1995).The droplet loses its initial kinetic energy or momentum afterimpingement with increase in impingement angle resulting in a weak filmpressure. Jayaratne & Mason, B. J., “The coalescence and bouncing ofwater drops at an air/water interface,” In Proc. of the Royal Society ofLondon. Series A, Mathematical and Physical Sciences, 280: 545-565(1964). Others have recognized that the spray distance controls thediffusive nature of the spray over the travel distance. Rukosuyev, etal. “Understanding the effects of system parameters of an ultrasoniccutting fluid application system for micromachining,” Journal ofManufacturing Processes 12/2: 92-98 (2010).

The present inventors have also recognized other limiting factors ofprior micro-machining efforts. Limiting factors include heat createdduring machining, which results from shearing of the metal by thecutting edge on the primary shear plane, and from friction at thetool-chip interaction. Effective penetration of the cutting fluid in thecutting zone is essential for longer tool life. In micro-machining, thewetting and penetration of the cutting zone by the fluid droplets iseasier because the machining parameters, e.g., depth of cut and feedrate are comparable to the droplet size (e.g. 10-50 μm) produced fromthe atomizer. Macro-machining has a tool-chip contact area that is muchlarger than droplet size, which renders the atomization-based cuttingfluid spray techniques used in micromaching ill-suited for achievingpenetration. Achieving effective penetration of a spreaded fluid filmthroughout the cutting region is important in order to provide bothcooling and lubrication effects. These effects are closely related tothe resulting film thickness, its pressure (i.e. lift force) between thetool and the chip, and its cooling coefficient and tribological effect.

Embodiments of the invention include systems and methods for producing athin film of an atomization-based cutting fluid spray that can providecooling and lubrication between a workpiece and a cutting tool duringmachining. Systems and methods of the invention create a thin film ofmicro-scale fluid droplets and direct that film to the cutting zone toimprove cutting dynamics and cooling. Methods and systems of theinvention are safer than high pressure and cryogenic techniques, aspreferred systems of the invention can use a gravity supply for a fluidtank. Spray is created with a small amount of fluid that is deliveredefficiently as a thin film.

Preferred embodiment methods and systems of the invention are alsoenvironmentally friendly. CO₂ is used in preferred systems for itsexcellent cooling processes, while the systems of the invention avoidthe need to provide high pressures. Systems of the invention mix CO₂with air in specific ratios and a mixing flow that is optimized.Pressure demands are reduced compared to prior systems that aredisfavored for their use of pure CO₂. Other inert gases with similarmolecular weights, and particularly molecular weights above that of O₂(molecular weight 32) can be used. Argon (˜40) is another option. Anadditional benefit of CO₂ is that it is inexpensive. Its recycling intopreferred processes of the invention also provides an environmentalbenefit.

A preferred system of the invention includes an ultrasonic-basedatomizer and a gravity-fed cutting fluid reservoir with a delivery tube.A nozzle section includes high-pressure gas delivery nozzle/tube at thenozzle-spray unit. The nozzle is configured to produce an axisymmetricco-flow jet produced of a high-velocity gas and micro-scale fluiddroplets. A flow evolution downstream position pattern is created todeliver a thin film at a tool-workpiece interface. Mixing tubes mix airand CO₂ in a common flow to produce a temperature that avoids formationof ice from water mixed in the concentrated cutting fluid. Deliverytubes are sized to maintain the same pressure for the different feeds.

Preferred systems of the invention can rely upon a gravity feed, whichavoids many safety hazards and design difficulties associated with highpressure systems. No pump is required to deliver cutting fluid. Gravityfeed of cutting fluid is possible, because preferred systems of theinvention utilize a very small amount of cutting fluid, e.g., up to0.167 L/min, as compared to conventional flood coolant, e.g., 1 L/min orabove, during machining at the macro-scale. With cutting fluid usagethat can be a tenth or less of the fluid used by flood systems, similaror better tool life and performance is achieved.

As the high-velocity gas, the present system utilizes significant amountof CO₂ from pressurized cylinder along with air. CO₂ helps to reduce thedispensing temperature as well as suppressing smoke from the cuttingzone. Smoke from burning cutting fluid is usually seen during machiningwith air alone or N₂ gas. Any inert gas alone or mixed with air could beapplied for reducing the dispensing temperature of the spray. However,the molecular weight or molar mass plays a vital role in diminishingsmoke from the cutting zone that is produced due to burning of cuttingfluid. For example, N₂ (molecular weight 28) was tested and producesundesirable smoke. With its higher molecular weight than that of O₂(32), CO₂ (44) helps in diminishing the smoke by forming a blanketaround the burning fluid and also by displacing the oxygen surroundingthe fluid. For this reason, other high molecular weight inert gases,e.g. Ar (˜40), can also be used.

A preferred system of the invention includes an ultrasonic atomizercontrolled by a generator. The exit delivery portion of the atomizershould be with the common chamber, however. The atomizer is within acommon chamber immediately proximate and behind a nozzle section of thechamber. Cutting fluid is provided to the atomizer from a gravity feedtank. If required, low velocity air inlets may be used to help flow frombehind the point where the cutting fluid is introduced. The atomizercreates droplets in a volume contained by a nozzle chamber.High-velocity gas is introduced from mixing unit that mixes air and CO₂to entrain the droplets in an entrainment zone at the outlet of thenozzle. Surrounding fluid droplets are entrained by a high-velocitycenter gas jet at the droplet nozzle outlet. The high-velocity centergas jet nozzle is co-axial with the chamber and the droplet nozzle.

In preferred embodiments, droplet velocity and gas velocity are set toproduce a droplet-gas co-flow with a core that focuses and produces athin film at a predetermined, and preferably optimized, distance fromthe nozzle when contacting a workpiece-tool interface. A preferredexample embodiment suitable for macro-machining of titanium alloys isconfigured to produce a combination of a 1.2 m/s droplet velocity and 26m/s gas velocity (at 35 mm distance from the gas nozzle) with a dropletspray behavior in terms of droplet entrainment angle and droplet densityacross the jet flare that provides a thin film. The fully-developedregion (i.e., self-similarity state) of the co-flow is at about 26 mmspray distance from the nozzle or above for the present gas nozzle exitdiameter of 1.6 mm. This configuration has been demonstrated to producea uniform thin fluid film for penetrating at the cutting interface. Inpreferred embodiments, dimensions and spray parameters are set toachieve a droplet entrainment angle in the range of 20-30°.

A preferred example embodiment nozzle for the system includes aconvergence of 4° for the droplet nozzle, which ensures atomizeddroplets can be entrained with the high-velocity gas. For the preferredexample, a convergence angle of 0.75° and exit diameter of 1.6 mm forthe gas nozzle were determined to develop the droplet-gas co-flow inself-similarity state before the spray impinges at the cutting zonewithin a feasible spray distance range (e.g., 25-40 mm) duringmachining. Operated with a combination of 1.2 m/s droplet velocity and26 m/s gas velocity (at 35 mm distance from the gas nozzle), thisproduced a fully-developed region (i.e., self-similarity state) of theco-flow at and after 26 mm spray/downstream distance when the exitdiameters of the gas nozzle and the droplet nozzle are set 1.6 and 18.8mm, respectively. This provides a ratio of the downstream distance tothe gas nozzle exit diameter at 16 or above. Preferred embodiments ofthe invention will now be discussed with respect to the drawings. Thedrawings may include schematic representations, which will be understoodby artisans in view of the general knowledge in the art and thedescription that follows. Features may be exaggerated in the drawingsfor emphasis, and features may not be to scale.

Referring now to FIG. 1A, a schematic representation of a preferredcutting fluid system 8 of the invention is shown. The system includes anultrasonic atomizer 10 that is controlled by a generator 12 to operateat resonant frequency at the tool tip. The atomizer 10 is held (or fit)within a common chamber 14 immediately proximate and behind a nozzlesection 16 of the chamber 14. The chamber 14 can be formed by a unitaryor multi-piece cover/enclosure 18. Cutting fluid 22 can be fed at thetip of the atomizer 10 from the delivery tube or from the behind throughthe body center of the atomizer 10, depending on its design. Cuttingfluid 22 is provided to the atomizer from a gravity feed tank 24, withflow being controlled by a valve 26. In experiments, this was controlledmanually, but can be automated. Measurements can provide feedback to acontrol system to control the flow of the cutting fluid according totool operation. Advantageously, this feed is low pressure and requiresno pumps or power. This enhances safety, reduces system complexity andreduces power consumption compared to many conventional systems. Lowvelocity air inlets 28, if required, provide air flow from behind thepoint where the cutting fluid 22 is introduced, which keeps the dropletsdispersed and flowing toward the droplet nozzle 16 for entrainment. Inone set of experiments the low velocity air inlets were used since theatomized droplets flowed at a low rate of 0.2 m/s. The low velocity airinlets can be used to increase this velocity. In other experiments, anatomizer atomized at a rate of about 20 mL/min, and the low velocity airinlets 28 were omitted. In the additional experiments, the chamber 18was modified to control air entrapping. The modifications in thisadditional experiment omitted the four air inlets 28 on the periphery ofthe chamber 18, eight small holes (3 mm diameter) at equal distance weremade at the same periphery location. Eight additional holes were formedin the backside of the chamber 18. These additional holes were locatedbetween the atomizer 10 and the chamber outer diameter point, and wereequal spaced in the radial direction.

FIG. 1B illustrates a system that is similar to FIG. 1A. The FIG. 1Bsystem adds a second valve 37, that serves as a cut-off valve. With thisarrangement, the valve 26 serves as a flow-rate valve and can be set andleft at a predetermined flow rate while the cut-off valve permitsturning flow on and off. The valve 26 can be adjusted to preciselycontrol the flow rate or be set at a particular flow rate, while thevalve 37 permits a quick stop of the flow. Preferred cut-off valvesinclude gate and butterfly valves, while the valve 26 is preferred to beglobe type.

FIG. 1C illustrates a system that is similar to FIG. 1B. The FIG. 1csystem delivers cutting fluid through the body of the atomizer 10instead of in front of (downstream) of the atomizer. Other parts of thesystem are the same, but the system of FIG. 1C can be easier to assembleas careful alignment of the fluid outlet at the tip of the atomizer 10is not required.

In each of the systems of FIGS. 1A-1C, the atomizer 10 creates alow-velocity droplet spray from the cutting fluid within the chamberthat is then entrained in an entrainment zone 30 of the nozzle section16 by high-velocity gas emitted from a high-velocity gas nozzle 32 fedfrom a high velocity gas inlet 34. Surrounding fluid droplets areentrained by a high-velocity center gas jet at nozzle outlet of the highvelocity gas nozzle 32. A focused atomized spray 36 is produced at apredetermined distance from the end of the nozzle section 32 and cancreate a uniform thin film at a workpiece-tool interface duringoperation.

In preferred embodiments, the high-velocity gas includes a substantialamount of CO₂, but in a low enough ratio to avoid formation of ice atthe outlet. In preferred embodiments the mixture achieves a spraytemperature of about 2° C. This temperature is selected to be slightlyabove the freezing point (0° C.) of water to avoid freezing. A range of˜1-4° C. is preferred to avoid freezing while maintaining a desirablecooling effect. High-velocity gas is introduced from mixing unit thatmixes air and CO₂ to entrain the droplets in an entrainment zone at theoutlet of the nozzle and produce the resultant jet spray.

The micro-size fluid droplets in the resultant jet spray are focused ata predetermined distance that permits direction of the spray at aworkpiece-tool interface to create a uniform thin film. Rather thanrelying upon an excess of fluid (i.e., flooding), the thin film isefficiently delivered and penetrates the interface to cool and lubricatethe work zone and tool.

Upon impingement, the spray-surface interaction results in one of thefour regimes: sticking, rebounding, spreading, and splashing, which canbe determined based on the non-dimensional Weber number, We(=ρu_(o)²d_(o)/σ), and a non-dimensional group,K_(y)(=u_(o)(ρ/σ)^(1/4)(ρ/μ)^(1/8)f^(−3/8), where, d_(o) is the dropletdiameter, u_(o) is the normal component of the droplet velocity, ρ isthe liquid density, σ is the liquid surface tension, μ is the liquiddynamic viscosity, and f is the frequency of the droplets impact. Thespreading regime (We≧10 and K_(y)≦17) is desired for effective wettingof the cutting zone, as has been recognized by others, but others havetypically relied upon flooding in an effort to provide effectivewetting. The FIGS. 1A-1C systems instead produce the resultant spray ofvery small droplets that achieves a uniform thin film.

FIG. 2 illustrates a preferred example embodiment mixing section for thehigh-velocity gas inlet 34. Example dimensions (diameters of feed pipes38 and 40) are shown in millimeters to illustrate ratios set to controlrespective volumetric pressures and flows of air and CO₂. CO₂ cools to asignificantly lower temperature as compared to air or N₂, when sprayedout of a nozzle from a pressurized tank. For example, when released fromcylinders, the temperature of the CO₂ gas is measured to be about −25°C. and −5° C. at 300 psi and 150 psi, respectively, whereas it is about18-20° C. for N₂ gas and air regardless of the pressure level.Therefore, compared to air and N₂, CO₂ gas is expected to provide morecooling. In addition, CO₂ gas is an industrial byproduct that can berecycled during machining. However, the water-mixed fluid droplets thatcome through the outer droplet nozzle will immediately form ice at theoutside body and the outlet of the high-velocity gas nozzle due to suchlower temperatures of CO₂ gas if pure CO₂ is used. As ice continuouslyforms, its size becomes large and it blocks the fluid droplets. Themixing arrangement avoids this problem, in the preferred example. Thepreferred example was tested experimentally with about 66% air and 34%CO₂ gas by volume mixed in the same flow line 34 and the resultingtemperature was measured to be about 2° C. during testing. The size ofthe delivery tubes 38 and 40 for these two gases were varied to maintainthis volumetric ratio while maintaining the same pressure level.

FIG. 3 is a partial perspective view of the nozzle section 16. A housing42 forms at its terminal end a droplet nozzle 44 of the nozzle section16. The high-velocity gas nozzle 32 is co-axial with the droplet nozzle44 and the nozzle section 16 housing. The inner diameter of the dropletnozzle provides a converging profile on its inner surface (best seen inFIG. 4) that promotes focusing. In an experimental device, the housing42 and nozzle 44 were formed separately and joined together, but thiscan also be a unitary structure. In an experimental device, the housing42 (common chamber) slide with its inner surface on the outer surface ofthe droplet nozzle 44, with the help of slot grooves. These two surfaces(of 42 and 44) were high precision surfaces. In the example experimentaldevice, the convergence slope (4°) is only given in the inner surface ofthe droplet nozzle 44.

FIG. 4 illustrates the cross-section of the FIG. 3 nozzle section 16,along with example dimensions and slopes of the converging innersurfaces of the high velocity gas nozzle 32 and the droplet nozzle 44.In the FIG. 4 design, there is a 4° convergence with 18.8 mm exit at thedroplet nozzle, ii) 0.75° convergence with 1.6 mm exit at the gasnozzle, iii) the gas nozzle exit is 5 mm inside of the droplet nozzleexit. (a, b, c, d, e can vary as convenience. In the above design, a=25mm, b=2 mm, c=15.2 mm, d=25 mm, e=10 mm). To produce a focused andhigh-pressure fluid droplet jet, the pressurized gas is deliveredthrough the high-velocity gas nozzle that adds its momentum to thedroplets. The spray distance during machining can be set at relativelyhigh values (e.g., about 30 mm) to allow for the flexibility of thenozzle unit and easy chip evacuation from the cutting zone. However, anydivergence of the spray jet is undesirable. A significant amount of thefluid droplets would miss the cutting region, and the droplet jet canlose its momentum or kinetic energy towards the cutting region. Sincethe pressurized fluid over the spray distance gradually diffuses fromits center streamline mainly due to the pressure difference from thesurroundings, the gas nozzle can be optimized by changing theconvergence slope (θ_(g)) to control the momentum and the coverage ofthe spray jet. With the inlet and outlet diameters of the droplet andhigh velocity gas nozzles having a convergence slope, θ_(m) of about 4°and θ_(g) of about 0.75°, respectively, as described, for a given spraydistance of about 25 and 35 mm, the diameters of the spray jet will beabout 0.95 and 0.68 mm, respectively. These sizes of the spray coverageprovided coverage of the cutting region during titanium machining wherethe tool-chip contact width is less than about ˜0.5 mm.

In experimental device the following were fixed dimensions: dropletnozzle: 4°±0.50 with 18.8±1 mm exit dia. Gas nozzle: 0.75±0.1° with1.6±0.1 mm exit dia. Droplet nozzle convergence was preferred so thatthe droplets tend to move toward the entrainment zone. In such case, thecenter gas can effectively entrain the droplets. Without convergence,the droplets in the boundary diffuse with atmospheric air immediatelyafter exiting from the droplet nozzle as the high-velocity gas cannoteffectively entrain the droplets that are comparatively at a largerradial distance. Artisans can adjust the fixed dimensions for particularapplications, and particularly, to adjust the distance from the nozzlewhere a thin film is optimally created. Generally, the exit diameter ofdroplet nozzle should be small enough to avoid interaction with therotating workpiece and/or stationary tool when the spray unit is setwith certain spray conditions such as impingement angle, spray distance.The droplet nozzle should be large enough to avoid droplet interactionbetween themselves and with the inner wall surface of the nozzle. Thelength of the nozzle section (section 16 in FIG. 1A) in the experimentaldevice is 30±1 mm. Generally, this length should be kept small enough toaccomplish the convergence and entrainment, while also permitting thegas nozzle to be set back within the droplet nozzle with a preferredposition 5±2 mm, which provides better entrainment of the droplets. Agoal in preferred embodiments is to set the dimensions for a particulardesign to achieve a droplet entrainment angle of 20-30° and provide adroplet free core at the nozzle exit that provides gradual mixing toachieve a fully developed flow at a predetermined distance away from thenozzle exit. At the beginning the fully developed flow point, thedroplet free core ends.

Dimension Relationships in FIG. 4:

The effect of dimensions is explained qualitatively, to provide guidanceto produce particular designs. The following parameters can be used byartisans to produce optimal designs for particular applications.

a: Length of enclosure 18, which connects the nozzle unit with theatomizer. Atomized droplets created from the tip should have a room tospread, otherwise the droplets would directly hit/interact the backsideof the gas nozzle 32. The spray pattern of the atomizer is alsopreferably configured to spread the atomized droplets around the highspeed gas nozzle to avoid such contact.

b and c: They have a relationship as determined by the convergence ofthe gas nozzle. Length ‘c’ should be selected so that the high-velocitygas nozzle is assembled within 16. Once ‘c’ is selected, ‘b’ can befound by the convergence angle as suggested.

d: It is droplet nozzle exit dia plus the wall thickness of the nozzle.Wall thickness should be at least 1 mm. A thicker wall, however, willincrease the size of the unit footprint, which may interact the rotatingworkpiece and/or the stationary tool for certain spray conditions usedduring machining.

e: Its dimension varies due to the assembly of the droplet nozzle andgas nozzle. If ‘c’ is selected very small, ‘e’ should be small forflexibility in assembly.

Experiments and Experimental Systems

Additional features and advantages of the embodiments discussed abovewill be apparent to artisans with reference to experiments andexperimental devices that were constructed and tested, as willadditional features and embodiments.

FIG. 5 shows a cutting system that includes a cutting tool system and acutting fluid spray system 8 consistent with FIGS. 1A-4 in a set up tolubricate and cool a tool/workpiece interface. A workpiece is not shownfor simplicity of illustration. The cutting tool include a tool shank 50that holds a tool 52. FIG. 5 is labelled with parameters to that aidunderstanding of the testing and performance of an experimental systemof the invention. Fluid film formation and its penetration, cooling andlubrication characteristics of an ACF spray system are influenced bypressure level of the droplet carrier gas and its type, fluid flow rate,droplet impingement angle, and spray distance. FIG. 5 shows therelationship between these parameters and fluid film formation.

An ultrasonic-based atomizer (Model VC5040AT from Sonic and Materials,Inc., CT) that vibrates at 40 kHz, was used in experiments to produceuniform fluid droplet size of about 50 μm at the maximum flow rate of 10L/hr (i.e. 166.67 mL/min). The atomizer was tightly placed inside aplastic transparent cover. The generator for the atomizer was locatedoutside the machine chamber for easy turn on/off. The cutting fluidreservoir was placed on top of the machine cover (outside the machine)so that the fluid can flow due to gravity. A fluid reservoir of 4-5gallon size can be used to machine for 16-32 hours at a flow rate of10-20 mL/min. A plastic tube was used to deliver the cutting fluid fromthe reservoir to the atomizer tip. Another four small plastic tubes wereintegrated with the cover behind the atomizer tip for supplying thelow-velocity air that assist in pushing the droplets through the nozzleunit.

The spray unit was placed inside the machine chamber (e.g. attached withthe lathe turret). The nozzle spray unit was placed in front of theatomizer tip at a distance about 30 mm and tightened inside the otherend of the plastic cover.

Machining Experiments

The inventors have determined that the wetting of the entire tool-chipcontact zone directly depends on the fluid flow rate. The inventors havealso determined that type of the droplet carrier gas can play asignificant role in reducing the temperature of the cutting zone, andpreferred embodiments use air and CO₂ to reduce the cutting zonetemperature.

In the Experiments, a Mori Seiki Frontier L-1 CNC lathe was used forturning experiments. The experimental arrangement is pictured in FIGS.6, 7A, and 7B. The experimental frame was constructed considering twolinear axes, a rotational axis and a replaceable wedge (depends ondesired impingement angle) so that the atomic cutting fluid spray unitof the invention 8 can easily be adjusted at a desired impingementangle, orientation, and spray and spot distances. FIG. 6 shows the spraysystem, 7A depicts the schematic of the frame and FIG. 7B shows an imageof the machining set-up with the frame. The free end of the metallicslab of the frame was fastened with the lathe turret, as shown in FIG.7C. The spray unit was directed on the tool rake face and its centerlinewas oriented along the major cutting edge (i.e. 60° with the work axis)for impinging the droplets in the direction of the cutting edge.

A cylindrical Ti-6Al-4V bar of size Ø175 mm×350 mm was used for turning.Triangular type uncoated microcrystalline carbide inserts ISO grade K313from Kennametal (TPGN220408) was used as tool material. The toolgeometry was set as follows: 5° rake angle, 11° clearance angle, 60°major cutting edge angle, 0.8 mm nose radius. The tool was placed with astandard Kennametal shank, which was then secured with a Kistler3-component force dynamometer (type 9121) to capture the cutting forcedata at a sampling frequency of 1 kHz through a National Instrument dataacquisition system (SCB-68) integrated with the LabVIEW software.Water-soluble cutting fluid S-1001 at 10% dilution was used as coolant.The thermo-physical properties of water and 10% S-1001 are presented inTable 1. Cutting fluid with higher viscosity and lower surface tensionwas found to be preferable for better lubricity.

A 2⁵⁻¹ fractional factorial design was employed in conductingexperiments. Table 2 lists the factor levels chosen for investigatingthe effect of five ACF spray parameters, i.e., fluid flow rate, spraydistance, impingement angle, type of mist carrier gas and its pressure.The range of the fluid flow rate 10-20 mL/min and the pressure level150-300 psi were selected to induce the spreading regime (We≧10 andK_(y)≦17) on the rake face. The velocity of the mist carrier gas, v_(g)in the gas nozzle was estimated to be about 26 m/s at 150 psi and 36 m/sat 300 psi when measured with an anemometer at 35 mm spray distance.Table 3 shows that spreading regime upon droplets impingement on therake face will occur under these conditions according to thenondimensional number W_(e) and group K_(y). The K_(y) values werecalculated considering 50% effective flow rate because the fluiddroplets were observed to be condensed about 50% during the experimentdue to their interactions with the outside of the gas nozzle. The airvelocity in the mist nozzle was kept fixed at 1.2 m/s. The spot distancewas set fixed at about 8 mm in all the tests.

During machining with conventional flood condition, coolant was directedon the rake using a standard delivery system at the flow rate and thepressure of about 1000 mL/min and 60 psi, respectively. The cuttingconditions were selected to be 80 m/min cutting speed, 0.2 mm/rev feedrate, and 1 mm depth of cut.

For all the cutting conditions, the tools were removed from the setupfirst at 4 min and then at 6 min to observe the progress of wear. Thetool thereafter was checked every one minute until the maximum flankwear land reached 0.6 mm according to the ISO standard. The maximum toolflank wear was measured using a Quadra-Check 300 optical microscope. Theproduced bulk chips were photographed by a digital camera and imageswere analyzed.

Tests were conducted using the ACF spray system for the conditionslisted in Table 2. Table 4 lists the results of cutting forces, toollife, and friction co-efficient at the tool-chip interface. The frictioncoefficient is calculated from the relationship between the tangentialand the feed force components for orthogonal cutting. Out of 16 tests inthe factorial design, 4 sets of the ACF spray conditions offer tool lifeof up to 10-11 min. For the same cutting conditions under flood cooling(test No. 17), the average tool life for two tests was found to be about7 min indicating that, with the ACF spray system, the tool life can beimproved up to 40-50% over flood cooling.

An analysis of the tool life data in Table 4 was done to determine thesignificant effects of the five ACF spray system parameters. It revealedthat the machining performances of Ti-6Al-4V are mainly influenced bytwo-factor interaction effects involving all five variables as shown inTable 5. Therefore, the main effects of the five variables must beinterpreted in together.

FIGS. 8A-8D show four two-way diagrams for the significant two-factorinteraction effects on tool life. FIG. 8A shows at a long spray distance(35 mm) of the liquid droplets, the tool life significantly improveswith the decrease in pressure level of the mist carrier gas, while at ashort spray distance of 25 mm, the tool life tends to decrease. FIG. 8Bshows that the tool life can be prolonged with the decrease in pressureof the gas at a high impingement angle. Delivery of the mist carrier gasat a low pressure (150 psi) is easier and more economical than thatcompared to a high pressure (300 psi) and thus this condition ispreferable, and can be achieved with excellent results through theinvention. FIG. 8C shows that N₂ gas offers longer tool life at a largeimpingement angle (35°), whereas air-mixed CO₂ offers longer tool lifeat a smaller impingement angle (25°). FIG. 8D shows that when the flowrate increases, long tool life can be obtained at a small impingementangle while a large impingement angle does not have significant impacton the tool life.

These results are interpreted to show that a combination of low gaspressure (or velocity), long spray distance, and high droplet flow ratefor both the gases applied in an ACF spray system results in longer toollife during titanium machining. The only exception is that, to obtain alonger tool life, the air-mixed CO₂ gas has to be impinged at a 25°angle whereas the N₂ gas is at a 35° angle.

Though both the gas types offer tool life similar (about 10 min), theair-CO₂ mixture is preferable during machining for a number of reasons.Overall chip breakability throughout machining is higher with the use ofair-CO₂ (90%) mixture as droplet carrier gas than with N₂ (40-50%). Ahigh rate of chip breakage is preferable as broken chips are less likelyto entangle and accumulate in the machining zone, and rub the machinedsurface. Unbroken accumulated chips also become obstacles to impingingliquid droplets leading to jet momentum reduction and reduction in theamount of fluid droplets in the cutting zone. N₂ gas can also cause afire hazard during Ti-6Al-4V machining. CO₂ gas is inexpensive, alsobecause it is a byproduct from industrial process, and its use inmethods of the invention provides environmental benefits as the CO₂ gasis recycled into the process.

The tools used in two tests under flood coolant condition (test No. 17)machined for about 6 and 8 min, respectively, before failure indicatingthat the average tool life is about 7 min. As the nose of the tool No. 1got chipped off after 6 min, and the wear exceeds 0.6 mm, the machiningwas stopped. The tools in these tests also produced heavy fire hazardsand smokes due to poor penetration of the cutting fluid at theinterface. Furthermore, the chips were rarely broken.

The results in Table 5 and FIGS. 8A-8D show that the combination of alow pressure gas, a long spray distance, and a high flow rate leads to alonger tool life in titanium machining. With the increase in pressure,the fluid film thickness decreases leading to a faster evaporation ofthe fluid before reaching the entire cutting edge. This can lead to thetool and the chip directly contacting each other, which causes higherfriction. As seen in Table 4, the average values of the frictioncoefficient are found to be smaller when gas is delivered at a lowpressure (150 psi). With the combination of a low pressure and a longspray distance (35 mm), a larger fluid film may be formed that helps inproviding sufficient cooling and lubrication effects in the entirecutting zone resulting in a longer tool life (FIG. 8A). In addition,air-mixed CO₂ gas prolongs tool life at a small impingement angle of25°. Since air-mixed CO₂ gas is preferable due to a number ofadvantages, a small impingement angle should be set during machiningwith the ACF spray system. When the flow rate increases, as seen in FIG.8D, long tool life can be obtained at a small impingement angle (25°).This is because a comparatively higher amount of cutting fluid (20ml/min) helps in improved wetting and spreading the fluid film over thecutting zone. A smaller impingement angle leads to comparatively higherkinetic energy (or lift force) towards the cutting region that furtherassists spreading the thin film.

The air-mixed CO₂ gas provides an initial temperature that is low (about2° C.), which encourages formation of broken chips. The hot chips comingfrom the cutting zone immediately come into contact with this gas andquench at that temperature, which leads to increasing the brittleness ofthe chip material. At the pressure level of 150 or 300 psi, these chipsare easily broken. In contrast, the initial temperature of impinging N₂gas is about 18-20° C., which does not promote breaking of the chips dueto the lack of brittleness.

Nitrogen gas also produced significant fire hazards in the machine zoneand produces smoke. As discussed above, the higher molecular inert gasesbehave differently and reduce the fire hazard. The air-CO₂ mixture helpsin diminishing the fire hazard and thus no fire hazard has beenobserved.

All the ACF spray system parameters such as pressure level and type ofthe mist carrier gas, mist flow rate, spray distance, impingement angleplay significant role in the machining performances such as tool lifeand chip formation. For some combinations of these parameters, the ACFspray system improves tool life up to 40-50% over flood cooling as shownby the results. Though both N₂ and air-CO₂ mixture offer about the sametool life, air-CO₂ effectively diminishes fire hazard in the cuttingzone while N₂ gas produces smoke by burning the mists at the elevatedcutting temperature. The use of air-CO₂ mixture in titanium turningoften produces broken chips, which do not interact with the finishedsurface, and are beneficial in terms of chip management. The ACF spraysystem of the invention is cost effective due to a significantly loweramount of cutting fluid consumption (10-20 mL/min) as compared to floodcooling (1000 mL/min or above).

Spray Experiments

Experiments were conducted using the experimental ACF spray unit tocharacterize droplet spray behavior. The inner gas nozzle was 5 mminside the droplet nozzle exit position to avoid divergence of droplets.Experiments using a 2² factorial design considering two gas velocitiesof 26 and 36 m/s, and two droplet velocities of 0.2 and 1.2 m/s wereperformed to study the effect of droplet velocity and gas velocity ondroplet spray characteristics including droplet entrainment zone (e.g.angle and distance) and flow development regions described by dropletdensity and droplet distribution that is shown in FIGS. 11 and 12. Thefluid flow rate was chosen to be 20 ml/min.

FIGS. 9A-9D are photographs of four different combinations of dropletvelocity, U_(d) and gas velocity, U_(g). The droplet entrainment angle,θ_(r), the direction at which the outer co-flow fluid droplets convergetowards the center axis for the conditions in FIGS. 9A-9D, are measuredto be about 43.26°, 24.44°, 55.0°, and 29.05°, respectively. Whendroplet velocity increases (conditions FIG. 9A vs. FIG. 9B or FIG. 9Cvs. FIG. 9D), the droplet impingement angle, θ_(r) becomes smaller. Asmaller value of θ_(r) allows droplets to be entrained slowly with acomparatively longer downstream distance resulting in a larger dropletentrainment zone. With an increase in gas velocity (conditions FIG. 9Avs. FIG. 9C or FIG. 9B vs. FIG. 9D), the droplet entrainment angleincreases leading to a smaller droplet entrainment zone. Theseparameters can be used to optimize the spray parameters for a givenapplication.

Experiments revealed a droplet-free zone at the center of the sprayafter its exit from the gas nozzle. After a certain distance thatdepends on the spray condition, the droplet and the gas merge anddroplets distribute uniformly across the jet flare. The flow developmentbehavior is characterized in FIGS. 11A & 11B, and also in FIGS. 11C &11D. When the gas exits at the center, it entrains the surrounding fluiddroplets creating a converging droplet entrainment zone around the gasnozzle for a certain distance. The resultant droplet-gas jet thendiverges, but the mixing or flow development continues. The entire flowdevelopment region starting from the gas nozzle exit point can bedescribed by three distinct regions: near-field (NF), intermediate-filed(IF), and far-field (FF), which are illustrated in FIG. 12A. Theseregions are usually characterized by a normalized axial position,x/d_(g), where d_(g) is the exit diameter of the gas nozzle. Typicalcross-sections A-A, B-B, and C-C of the jet flare at the NF, IF, and FFregions are shown in FIGS. 12B-12D, and the full spray pattern in FIG.12A. In the NF region, a potential core is observed with absence of theouter co-flow medium. In contrast, in the FF region, no potential coreis observed as the mixing is fully-developed (i.e. ‘self-similar’ state)and hence, the droplets are uniformly distributed throughout the jetflare. The IF or transition region that lies between these two regionscontains a few droplets as it approaches to the FF region.

A liquid dispensing into still ambient air or parallel moving air/gas,defines an FF region is approximately x/dg≧25. See, Rukosuyev, M., etal., “Design and development of cutting fluid system based on ultrasonicatomization for micro-machining,” Transaction of the NAMRI/SME 38:97-104 (2010); Fellouah, H., et al., “Reynolds number effects within thedevelopment region of a turbulent round free jet,” International Journalof Heat and Mass Transfer 52: 3943-3954 (2009).

The experimental device used 1.6 mm, and hence, the self-similar state(i.e. distance between the gas nozzle exit and the FF region) isexpected to fall at 40 mm or beyond. However, the fully-developed or FFregion was 24-35 mm, which is smaller than predicted by past studies.The early flow development of the self-similar state in the experimentstesting embodiments of the invention could be attributable to thedensity of the center gas (air-CO₂) being lower than the outer co-flowgas (droplet). Also, when gas velocity decreases, the distance betweenthe gas nozzle exit and the FF region becomes shorter. The experimentssuggest that the downstream distance for the self-similar state, wherethe flow becomes asymptotic, gets smaller with the reduction in gasvelocity.

The experiments show that droplet and gas velocities in an ACF spraysystem influence the droplet entrainment mechanism and the droplet-gasmixing behavior at the center jet. A combination of a higher dropletvelocity (1.2 m/s) and a lower gas velocity (26 m/s), among four sprayconditions observed, provides the best spray condition in terms ofdroplet entrainment angle.

The behavior can be modeled to provide a theoretical relationship ofdroplet and gas velocities with droplet entrainment angle andentrainment zone, and their influence on droplet density anddistribution across the jet flare at three different regions (i.e. NF,IF, and FF). When a high-velocity fluid is dispensed into a stillatmospheric air or to a low-velocity parallel moving fluid, entrainmentof the outer fluid into the inner fluid takes place. As thehigh-velocity fluid jet flows at a dynamic pressure, its static pressurereduces according to Bernoulli's principle. This causes a pressuredifference between the jet domain and the surrounding fluid/air.Moreover, if a compressible fluid like gas is dispensed (exit from anozzle), it immediately expands in the radial direction. The pressuredifference and the gaseous nature of the dispensing fluid causeentraining of the outer surrounding gas or air to flow in and mix withthe center gas. Due to differences in fluid properties and flow dynamicsbetween the center jet and the outer droplets, one fluid diffuses intothe other and vice versa. The slow-moving droplets cause aerodynamicdrag to the high-velocity gas that results in deceleration of the gas,and the momentum lost by the high-velocity gas is received by thedroplets. Their momentum gradually evolves to an equilibrium state (i.e.fully-developed) as the jet moves forward. A fully-developed flow isdesirable. In FIG. 12A, the point where the far-field region starts isideal point for a tool-surface interface to be located (also the pointwhere the central droplet free core ends). As a result, the velocity ordynamic pressure of the center jet, which is assumed to follow aGaussian profile, and decays with respect to the downstream position asseen in FIGS. 11A-11D and 12A-12D.

In the experiments, it was also shown that an increase of the dropletvelocity (e.g., up to 1 m/s) with a decrease of the gas velocity (e.g.,36 m/s to 26 m/s when measure at about 35 mm downstream distance fromthe gas nozzle 32) produce a better spray. The droplets-gas co-flowdevelop in short distance. This provides a potential core that does notcontains droplets disappear early. The droplet entrainment angle becomessmaller with this combination (compare FIG. 9B with other three 9A, 9C,9D). As the decay of a high-velocity spray jet is evident over thedistance, it is desirable to use this high-momentum droplets-gas mixedjet for achieving effective cooling and lubrication in the machiningapplication as soon as they achieve an equilibrium state. An exampleoptimization was conducted in the experiments. Specifically, with theexample experimental geometries, co-flow development was found to takeplace at a 26-35 mm spray distance (from the gas exit 32), which isabout 16-22 times of the gas nozzle exit diameter of 1.6 mm. In theexperimental nozzle system consistent with FIG. 1A, and illustrated inFIG. 4 the example exit diameter of the nozzle section 44 was 18.8 mm(with outer dia of 25 mm). Variations are permitted. Factors that affectthe size of the diameter is that it should be small enough to permitpositioning close to a cutting surface during machining. It also shouldbe large enough to avoid detrimental droplet agglomeration which occurswhen droplets will hit the nozzle inner surface. In the experimentaldevice, the exit of the gas nozzle 32 was placed 3-5 mm inside of thenozzle section, which provided better control of the droplets in view ofthe example droplet nozzle size. The goal in choosing this placement isto promote the droplets to enter the droplet entrainment zone before thedroplets exit the nozzle.

To estimate the droplet entrainment angle, the following assumptions aremade about the nature of the droplets:

Fluid droplets inside the droplet nozzle uniformly mix with the ambientair and create a homogenous droplet vapor. It appears to be a singlegaseous phase, but different from the centerline gas jet in nature;

Fluid droplets do not interact each other during flow (non-condensing).

The fluid droplets are uniform in size.

Gravitational force acting on the droplets is ignored.

When a high-velocity gas is dispensed from a nozzle, the pressuredifference between the jet and the surrounding rest or co-flow jetincreases with the increase in its velocity. The Gaussian mean velocityprofile shown in FIGS. 12A-12D implies that the velocity or dynamicpressure of the jet is the largest at the center producing to the leaststatic pressure. In ACF spray system, the velocity of a static singledroplet, U_(gs) towards the jet center due to a reduced static pressurecan be calculated from Bernoulli's principle:

$\begin{matrix}{{{P_{d} - P_{gs}} = {\frac{1}{2}\rho_{d}U_{gs}^{2}}},} & (1)\end{matrix}$

where, P_(d), P_(gs), ρ_(d) refer to the static pressure of the dropletsin the droplet nozzle, the static pressure of the gas at the jet center,and the droplet equivalent density of the droplet vapor, respectively.The pressure difference, (P_(d)−P_(gs)) can be directly measured from apiezometer. The static pressure at the jet center, P_(gs) decreases withan increase in gas velocity. For a given control volume consisting of acircular ACF nozzle unit, an equivalent density of the droplet vapor canbe calculated using mass conservation law:

$\begin{matrix}{{\rho_{d} = \frac{{\rho_{f}{\overset{.}{V}}_{f}t} + {\rho_{a}\left( {V_{N} - {{\overset{.}{V}}_{f}t}} \right)}}{V_{N}}},} & (2)\end{matrix}$

where, ρ_(f) and ρ_(a) are the fluid and air densities, {dot over(V)}_(f) is the volumetric fluid flow rate, t is time, V_(N) is thecontrol volume in round nozzle.

When a single droplet that is entrained in the droplet entrainment zone(formed around the potential core immediately after the gas nozzle exit)approaches to flow in the center at an angle, θ_(gs) due to the staticpressure drop, the dynamic pressure of the jet that flows at a highvelocity, U_(g) pulls off the droplet in its flow direction at avelocity, U_(gr) at the outer contour of the jet. The flow of thedroplet due to velocities, U_(d), U_(gs), and U_(gr) can be representedby the velocity diagram of FIG. 10. The resultant droplet entrainmentangle, θ_(r) of a co-flowing droplet can be expressed as:

$\begin{matrix}{{\theta_{r} = {\tan^{- 1}\left( \frac{U_{gs}}{U_{d} + U_{g\; r}} \right)}},} & (3)\end{matrix}$

where U_(gr) is the velocity at the outer contour of the jet, and itsvalue is obtained as:U _(gr) =k _(x) U _(gx),  (4)

where, k_(x) is a proportionality constant, and U_(gx) is the local jetvelocity in the x-axis. At the nozzle exit, x=0, the value of U_(gx) isapproximately equal to U_(g); however, it decreases with the increase indownstream distance as the velocity of the jet decays. Also, for acompressible non-viscous fluid flow, k_(x)≈1 at x=0, because thevelocity at the jet center is approximately equal to the velocity at theouter contour.

Equation 3 states that the droplet entrainment angle can be influencedby both gas velocity and droplet velocity. A higher droplet velocity,U_(d) governs a droplet to follow at a smaller angle, θ_(r) with respectto the jet axis. However, the dependence of gas velocity, U_(g) on thevalue of θ_(r) is a bit complex. With the increase in gas velocity,U_(g), both U_(gr) and U_(gs) increase. Note that, if the value of U_(g)(or dynamic pressure) of the jet increases, the value of U_(gs) alsoincreases due to a comparatively larger static pressure drop,(P_(d)−P_(gs)). Predicting the influence of U_(g) requires knowledge ofthe value of the static pressure of the jet, P_(gs).

The expression for the droplet entrainment angle, θ_(r) in Eqn. (3) isobtained considering a single droplet in the droplet entrainment zone.However, when considering a number of droplets around the center jet,the droplet that is close to the gas jet contour will move faster thanthe one that is farther away. Therefore, the value of θ_(r) estimatedfrom the above relationship for a co-flow jet may not be accurate.

The density and distribution of droplets across the jet flares atlocations A, B, and C for three different regions: NF, IF and FF, varyas shown in FIGS. 12A-12D due to gradual mixing between the droplets andthe gas with respect to the downstream distance. Across the jet flare atcross-section A-A, the resultant jet of the droplets and the gas doesnot significantly diverge. Diffusion between the droplets and the gas isminimal. As a potential core (i.e. no presence of the droplets) isdistinctly observed at the center and the combined jet has a smallerouter contour, the number of droplets for a given volume (i.e. dropletdensity) becomes too high. Across the jet flare at C-C, the diffusion isfully-developed, and the jet contains a larger contour because ofdivergence. As a result, the droplets uniformly distribute and thedroplet density becomes smaller. At the cross-section B-B, the number ofdroplets across the jet flare may not be significant, but the size ofpotential core becomes smaller. If for the locations A, B, C, a smalllength, Δx is considered in both directions along x-axis, the averagedroplet density across the jet at the respective locations can beestimated as:

$\begin{matrix}{\rho_{dA} = \frac{N}{2\pi\;\Delta\;{x\left( {r_{jA}^{2} - r_{p\; A}^{2}} \right)}}} & \left( {5a} \right) \\{\rho_{d\; B} = \frac{N}{2\pi\;\Delta\;{x\left( {r_{jB}^{2} - r_{iB}^{2}} \right)}}} & \left( {5b} \right) \\{{\rho_{d\; C} = \frac{N}{2\pi\;\Delta\;{xr}_{jC}^{2}}},} & \left( {5c} \right)\end{matrix}$

where, N is the number of fluid droplets depending on the flow rate andatomizer frequency, r_(jA), r_(jB), r_(jC) are radii of the jet boundarylayer at the locations A, B, C, respectively, and r_(pA), r_(iB) areradii of the potential core and the intermediate core at the locations,A, B, respectively. The jet boundary layer suffers from an eddy(turbulence) effect due to entrainment of atmospheric air. As such,average values of r_(jB) and r_(jC) should be taken for the intermediateand far-field regions. Eqns. 5(a)-(c) suggest that the average dropletdensity is significantly higher at location A followed by medium atlocation B, and the least at location C.

The droplet vapor is assumed to behave like one medium as anon-condensing gas. However, the fluid droplets may interact, especiallywhen they come close or touch each other. This situation can occur intwo ways: during entrainment in the droplet entrainment zone and duringmixing within the NF (i.e. near-field) region. For a set of droplet andgas velocities, if the droplet entrainment angle increases, then thesize of the droplet entrainment zone decreases due to a smallerdownstream distance. In such a case, the droplet density becomes higherand the distance between the droplets may reduce. If two or moredroplets touch each other, they will result in condensation and willform, comparatively, a larger droplet. Also, during mixing within the NFregion, the entire amount of droplets usually passes through a smallerjet flare (for example, at cross section A-A in FIGS. 12A & 12B). As thedroplets again become closer, there is an increased risk of condensationeffect. When droplet size becomes larger, it can influence filmformation behavior, and hence will affect the cooling and lubricationcharacteristics during machining.

To avoid condensation of the fluid droplets in the entrainment zone andnext in the mixing region (i.e. near-filed and intermediate-field), itis preferable to allow the droplets to be entrained slowly and graduallyuntil the intermediate region. In such a case, the radius of the jet atthe NF and the IF regions will increase and the droplet density willdecrease that will protect the fluid droplets from condensation. FIGS.11A and 11B together and FIGS. 11C and 11D together respectivelyillustrate schematic diagrams for a smaller and a larger dropletimpingement angles. A smaller droplet entrainment angle leads toincrease the size of the droplet entrainment zone as well as the radiusof the droplet-gas resultant jet, r_(j). Using Eqn. (5a), it can be seenthat the average droplet density for a larger value of angle θ_(r) (say,θ_(r1)) at the same location A of the potential core will be higher thanthat for a smaller value of θ_(r) (say, θ_(r2)). Thus, if θ_(r1)>θ_(r2),then ρ_(d1)>ρ_(d2) since r_(j1)<r_(j2). This condition can be achievedby controlling both the gas velocity and the droplet velocity thatcontrol the droplet entrainment angle (Eqn. (3)). During machining withACF spray system, the center of the jet is directed toward the cuttingzone. A fully-developed flow achieves a uniform fluid film forpenetrating into the tool-chip interface.

While specific embodiments of the present invention have been shown anddescribed, it should be understood that other modifications,substitutions and alternatives are apparent to one of ordinary skill inthe art. Such modifications, substitutions and alternatives can be madewithout departing from the spirit and scope of the invention, whichshould be determined from the appended claims.

Various features of the invention are set forth in the appended claims.

TABLES

TABLE 1 Thermo-physical properties of 10% S-1001 cutting fluid andwater. Thermal Surface tension Density Viscosity conductivity Fluid(mN/m) (kg/m³) (cP) (W/mK) Water 72 1000 1.01 0.58 10% S-1001 41 10031.22 0.53

TABLE 2 Factor levels for the ACF spray system parameters Parameters Low(−) High (+) (x₁) Gas pressure (psi) 150 300 (x₂) Fluid flow rate 10 20(ml/min) (x₃) Impingement 25 35 angle (°) (x₄) Spray distance 25 35 (mm)(x₅) Mist carrier gas N₂ Air-CO₂ type

TABLE 3 Shows values of We and Ky at given ACF spray conditions. ImpactGas Normal Angle, velocity, velocity, Ky at x₂* x₃ (°) v_(g) (m/s) u_(o)(m/s) We 10 20 25 26 10.99 144.17 3.85 2.97 35 26 14.91 265.55 5.23 4.0325 36 15.21 276.39 5.34 4.11 35 36 20.65 509.11 7.24 5.58 (*calculatedat 50% effective flow rate)

TABLE 4 Experimental results for different ACF spray conditions andflood coolant condition. Cutting force (N) at 1 min Tool life FrictionTest RO x₁ x₂ x₃ x₄ x₅ Thrust Feed Tangential Resultant (min)coefficient 1 9 − − − − + 140.28 149.42 433.85 479.83 8 0.50 2 10 + − −− − 116.19 156.81 401.65 446.55 10 0.56 3 6 − + − − − 145.64 151.96480.96 525.00 6 0.47 4 13 + + − − + 126.20 137.95 431.41 470.18 9 0.47 52 − − + − − 123.78 124.08 406.33 442.51 8 0.45 6 1 + − + − + 123.50130.39 443.47 478.46 7 0.44 7 14 − + + − + 133.29 133.88 415.43 456.36 80.48 8 4 + + + − − 135.21 147.86 439.08 482.63 10 0.49 9 16 − − − + −115.09 109.68 415.26 444.66 8 0.40 10 8 + − − + + 139.50 151.61 453.02497.67 8 0.49 11 12 − + − + + 134.52 144.51 448.24 489.80 10 0.46 127 + + − + − 149.50 158.03 442.08 492.70 8 0.52 13 15 − − + + + 133.96146.80 452.90 494.58 9 0.48 14 11 + − + + − 129.17 137.52 418.77 459.317 0.48 15 5 − + + + − 126.93 149.29 427.87 470.61 11 0.51 16 3 + + + + +140.13 141.28 431.86 475.50 6 0.48 17 Flood coolant 121.63 126.82 412.15448.04 7 0.46 condition

TABLE 5 Values of significant effects. Response Factors Effect Tool lifePressure-Spray distance −1.88 Pressure-Impingement angle −1.13Impingement angle-Gas type −1.13 Flow rate-Impingement angle 0.63

The invention claimed is:
 1. An atomizing cutting fluid system,comprising: a common chamber terminating in a shaped droplet nozzle andincluding a nozzle section immediately behind the shaped droplet nozzle;an atomizer that creates spray directly within the common chamber behindthe nozzle section; a cutting fluid supply line to provide cutting fluidto the atomizer; and a high velocity gas nozzle within the nozzlesection and behind the droplet nozzle configured to provide a highvelocity gas to entrain the flow of droplets, wherein the nozzle sectionand droplet nozzle are configured to produce a fully developeddroplets-gas flow at a predetermined distance from the droplet nozzle.2. The system of claim 1, further comprising a high velocity gas mixingsection that mixes air and CO₂ in a ratio that produces a temperatureslightly higher than the freezing point of water to the high velocitygas nozzle.
 3. The system of claim 1, further comprising a gravity fed,pumpless cutting fluid supply tank supplying cutting fluid to thecutting fluid supply line.
 4. The system of claim 1, wherein the highvelocity gas nozzle and the droplet nozzle are co-axially disposed. 5.The system of claim 1, wherein the droplet nozzle and the nozzle sectionprovide a ˜4° convergence with ˜18.8 mm exit at the droplet nozzle,there is a ˜0.75° convergence with ˜1.6 mm exit at the high velocity gasnozzle and the high velocity gas nozzle exit is ˜5 mm behind the dropletnozzle.
 6. The system of claim 1, used in a method to machine titaniumalloy and disposed at the predetermined distance to create a thin filmon a tool substrate and effectively penetrate that film into thetool-chip interface.
 7. The system of claim 6, further comprising asystem controller, wherein droplet flow velocity and gas velocity arecontrolled by said system controller to set a droplet entrainment anglethat produces a focused droplet and entrained flow at the predetermineddistance.
 8. The system of claim 7, wherein the droplet flow velocity,droplet size, and gas velocity are controlled to allow droplets to beentrained and mixed gradually until achieving a fully-developed flowregion.
 9. The system of claim 1, further comprising a high velocity gasdelivery supply that delivers high molecular weight inert gas to thehigh velocity gas nozzle.
 10. The system of claim 1, further comprisingair inlets disposed behind the atomizer to disperse and create a flow ofdroplets created by the atomizer.
 11. The system of claim 1, whereinsaid cutting fluid supply line provides cutting fluid in close proximityand in front of a tip of the atomizer.
 12. The system of claim 11,further comprising a valve to control delivery of cutting fluid fromsaid cutting fluid supply line.
 13. The system of claim 12, wherein saidvalve comprises a plurality of valves including a shut-off valveupstream of a flow control valve.
 14. The system of claim 1, whereinsaid cutting fluid supply line provides cutting fluid through a bodycenter of the atomizer.
 15. The system of claim 1, wherein said cuttingfluid is supplied via gravity flow without any pump.
 16. A cuttingsystem including a cutting fluid system of claim 1, the cutting systemincluding a tool, the cutting fluid system being arranged to deliver athin film at an interface between the tool and a workpiece.
 17. Thesystem of claim 1, wherein the high velocity gas comprises a mixture ofair and CO₂ in a ratio that produces a droplets-gas flow having atemperature in the range of ˜1-4° C.